In the process of laminating or molding structures made from plastic composite materials such as fiber-reinforced resins, care must be taken to eliminate internal voids resulting from the entrapment in the material of air pockets or of gas bubbles formed by the vaporization of moisture, solvents and other residual volatile components.
Air pockets can be formed during the mixing and pouring of the material. Gas bubbles occur mainly during heating and curing phases of the processes.
Until now the most common and practical method for eliminating internal voids has been to enclose the mass of material being molded or laminated into a fluid-tight envelope, typically a bag of plastic sheet material connected to a vacuum pump. Air pockets and gas bubbles which may form in the plastic material are sucked out by the vacuum pump during exposure to internal pressure of an autoclave, and during the pressing, molding and curing cycles. The envelope also acts as a barrier against reentry of air or other ambient gas after the vacuum pull is interrupted or terminated during handling of the material between cycles.
The plastic bag or other fluid-tight envelope is a cumbersome element that, not only impairs the convenient manipulation of the material throughout the various manufacturing cycles, but can also result in the formation of flaws over the surface of the manufactured articles due to folds and creases in the envelope material.
The instant invention results from attempts to palliate the above-described inconvenience and problems by avoiding the need of any vacuum bag or other fluid-tight barrier between the composite material and the surfaces of the laminating or molding implements.